Extensive efforts have been undertaken for the development of thermal barrier coatings (hereafter, referred to as “TBCs”) for use in various applications on metallic substrates. Various metallic substrates require thermal protection. By way of example, superalloy substrates utilized in gas turbine aircraft engines and land-based industrial gas turbines require thermal protection. Further, steel substrates for the exhaust system of internal combustion engines require thermal protection. Currently, the use of TBCs can potentially allow a reduction of metallic substrate temperatures by as much as approximately 160° C., thus increasing the lifetime of a metallic substrate by up to four times.
A typical TBC system requires a bond coat, such as overlay MCrAlY coating or diffusion aluminide, that protects the metallic substrate from oxidation and corrosion, and a top coat that reduces heat flux into the component. Top coats are invariably based on ceramic materials. Yttria-stabilized zirconia (YSZ) is frequently utilized because of its high temperature stability, low thermal conductivity and good erosion resistance. YSZ is also preferred because of the relative ease with which it can be deposited by different techniques such as thermal spraying (plasma, flame and HVOF) and electron beam physical vapor deposition (EBPVD) techniques.
TBCs applied by atmospheric plasma process (APS) are built by flattened particles of a ceramic material, and contain laminar pores and microcracks between the particles. This microstructure is an important factor contributing to the thermal barrier properties of YSZ coating because these pores and cracks can dramatically reduce the thermal conductivity of the coating as compared to the bulk material, as well as alleviate thermally induced stress and thus increase thermal shock resistance.
It is important for a TBC to preserve its low thermal conductivity throughout the life of a coated component. However, plasma-sprayed TBC layers are often in inherently thermodynamically metastable state because of the rapid quenching of the molten particles on a substrate during the spray process. Upon exposure to high service temperatures, transformation toward an equilibrium state occurs and intrinsic thermal instability of the material microstructure results in TBC sintering and porosity degradation and thus deterioration of thermal barrier properties of the coating.
EBPVD YSZ coatings have a fine columnar microstructure that is better capable of accommodating a mismatch between the thermal substrate and coating compared to plasma-sprayed layers. As a result, EBPVD TBCs are often employed in some of the most demanding and advanced applications. However EBPVD coatings are rather costly, and thus not economically viable for some applications. Further, their columnar structure provides paths for penetration of corrosive species through the coating thus decreasing corrosion resistance of the overlay.
EBPVD and Plasma spray deposition methods are line-of-sight processes that are suitable for the coating application to visible areas of a substrate. Therefore, the substrates which can be coated by these spray methods are limited to simple geometries or substrates only requiring a coating on the external features.
Slurry-based coating deposition processes may also be utilized. Slurry-based TBC coatings and their application have been investigated many times in the past already. A slurry process comprises preparing an aqueous or solvent-based slurry, applying the slurry to the substrate, drying and heat treating or sintering to obtain a coating layer. This process could be repeated to form a coating of desirable thickness. However current developments in the art still do not resolve concerns associated with slurry-derived TBC application, such as creating a coating that is sufficiently thick to provide required thermal insulation (that is more than at least 300-350 microns), as well as to prevent coating excessive shrinkage during drying and cure of the applied layers that results in coating bonding problems to the surface of a coated part and eventual coating spallation.
Sol-gel techniques are known to generally deliver good coating—substrate adhesion. However, they cannot provide practical ways to achieve a coating thickness higher than 10-50 microns that is not sufficient for thermal insulation.
In view of the several shortcomings of current TBC technology, there remains an unmet need for TBCs that can withstand high service temperatures and retain their structural integrity. As will be discussed, the inventors herein have identified the problem of coating degradation and have remedied the problem in accordance with the present invention in order to provide a protective coating exhibiting thermal and environmental barrier properties suitable for high temperature applications.